1. Field of the Invention
The invention pertains to a flux composition and to a corresponding method for soldering, for example, a semiconductor chip or a chip carrier module to a printed circuit board. Specifically, the invention pertains to a flux composition and method for soldering high density C4 arrays.
2. Background
Fluxes play an important role in the procedures used to mount electronic components onto printed circuit cards and printed circuit boards (both of which are hereinafter generically referred to as printed circuit boards or PCBs). For example, one method for directly mounting a semiconductor integrated circuit device (hereinafter denominated a semiconductor chip or just a chip) onto a PCB is, for example, to form regions of solder, e.g., solder balls, on contact pads on the circuit-bearing surface of the chip. Such solder regions may also be formed on corresponding contact pads on the PCB. A flux is then applied to the solder regions on the chip and/or to the corresponding contact pads and/or corresponding solder regions on the PCB in order to remove oxide layers which may have formed on these solder regions or contact pads and to achieve increased wetting of the contact pads by the solder regions. Thereafter, with the circuit-bearing surface of the chip facing the PCB, the solder regions on the chip are brought into contact with the corresponding contact pads or solder regions on the PCB, and the resulting assembly is heated in order to melt, and thereby reflow, the solder regions on the chip and/or on the PCB. Upon cooling and re-solidification, the resulting solder connections between the chip and the PCB are typically encapsulated in an encapsulant, e.g., an epoxy resin encapsulant, to relieve any strains which may be engendered by a difference between the coefficient of thermal expansion (CTE) of the PCB and the CTE of the chip.
In a manner similar to that described above, one method for mounting a module, e.g., an organic module or a ceramic module, bearing semiconductor chips (hereinafter denominated a chip carrier module or just module) onto a PCB, involves forming, e.g., screening, regions of solder onto contact pads on the non-chip-bearing surface of the module. Such solder regions may also be formed on corresponding contact pads on the PCB. A flux is then applied to the solder regions on the module and/or to the corresponding contact pads and/or corresponding solder regions on the PCB. Thereafter, with the non-chip-bearing surface of the module facing the PCB, the solder regions on the module are brought into contact with the corresponding contact pads or solder regions on the PCB and the resulting assembly is heated in order to melt, and thereby reflow, the solder regions on the module and/or on the PCB. In general, the magnitude of the difference between the CTE of the module and the CTE of the PCB is relatively small, and therefore the resulting solder connections between the module and the PCB need not be encapsulated in an encapsulant.
If the module of interest has electrically conductive pins extending from the non-chip-bearing surface of the module, then the module is mounted onto a PCB by, for example, initially positioning the module over the top (i.e., the circuit-bearing) surface of the printed circuit board and inserting the electrically conductive pins of the module into corresponding, copper plated through holes (PTHs) extending through the thickness of the PCB. Then, the PCB and the module are placed on a conveyor, which passes the PCB and module over a fluxing wave or flux sprayer, which serves to impinge liquid flux onto the bottom surface of the PCB and into the PTHs. This flux is wicked up into the PTHs, and thus flux is applied to both the walls of the PTHs and to the pins extending into the PTHs. Thereafter, the conveyor passes the PCB and module over a solder wave, which serves to impinge liquid solder onto the bottom surface of the of the PCB and into the PTHs. This liquid solder is also wicked up into the PTHs, filling the PTHs and, upon cooling and solidification, serving to encapsulate the pins within the PTHs.
One of the most important aspects of the above-described chip-mounting and module-mounting procedures is the choice of flux. That is, as noted above, the flux serves to remove any oxide layers which may have formed on the solder regions, contact pads, pins or PTHs and to increase the wetting of, for example, contact pads by solder regions. In most instances, at the completion of the soldering process, use of the commonly available fluxes results in ionic residues remaining on the solder regions, contact pads, pins or PTHs. Such ionic residues are undesirable because they lead to corrosion of circuitry and to short circuits. Consequently, if formed, such ionic residues must be removed, e.g., cleaned with water, after the completion of the soldering process.
The solder connections formed between a chip and a PCB or between a pinless module and a PCB, as described above, have relatively small heights, e.g., 4 mils, and therefore the spacing between a chip or pinless module and its PCB is correspondingly small. This is significant because it implies that it would be very difficult, if not impossible, to clean away any ionic residues remaining on the solder regions and/or contact pads after the completion of the soldering process. In addition, in the case of a pinned module, while corresponding ionic residues are readily cleaned with water, one must then deal with the environmental hazards posed by the resulting contaminated water.
Significantly, those engaged in the development of fluxes and soldering processes for mounting chips and modules onto PCBs have sought, thus far with little success, fluxes which leave essentially no ionic residues on solder regions, contact pads, pins or PTHs at the completion of the corresponding soldering processes.
High density arrays present particular problems with regards to fluxes. Whereas typical low density arrays may have 32 contact points, high density arrays may have 300-2000 or even more contact points. Very large, high density C4 arrays present a problem not experienced in lower density arrays in that the total oxidized surface area of these C4 arrays deplete the usual active fluxing component before reflow is complete and consequently a significant number of non-wets results. Typically, increasing the active component concentration is not viable because of increased residues which increase the potential for electrical failure and decreased underfill adhesion.
Others solve this problem in several ways. The residues formed but not removed by cleaning may be made compatible with the underfill adhesive in order to promote adhesion by the addition of other components into the flux. However, this may result in material that may restrict flow of the underfill in the channels formed by the solder joints, which can result in void formation. Alternatively, the flux residues may require cleaning. Tools to perform adequate cleaning in spaces 0.0025xe2x80x3 to 0.004xe2x80x3 high are difficult to find and the process must be very thorough to remove residues which can cause subsequent delamination or lead to corrosion or dendritic growth from moisture exposure. Combined flux/underfill formulations have also been used, but significant challenges are still evident in promoting good solder joint formation, adequate adhesion and resistance to long term stress conditions. The proposed flux offers the simplest approach to provide optimum chip joint formation, underfill adhesion, and long term stress performance.
This invention succeeds where previous efforts have failed by providing a residue free flux composition suitable for high density arrays.
This invention provides advantages that were not previously appreciated by providing a flux effective for connecting high density arrays to printed circuit boards without leaving a residue.
In summary, the invention provides a flux composition which allows a higher concentration of a dicarboxylic active ingredient because of a unique property which allows it to volatilize more readily from the oxidized surface of the very dense arrays. The active fluxing component reacts with surface solder oxides at extended dwell times of 120 to 240 seconds above solder reflow temperature at peak temperatures in the range of 210xc2x0 C. to 245xc2x0 C. and preferably 220xc2x0 C. to 245xc2x0 C. for reflow of eutectic tin/lead solder, to enable solder joint information. The flux composition of the present invention may be used with other solder alloys at differing dwell time and peak temperature conditions.
The flux composition of the present invention is suitable for use with high density arrays and comprises a dicarboxylic acid in an amount sufficient to react with the oxidized surface area in a high density array, a first organic solvent, and a second organic solvent having a higher evaporation temperature than that of said first organic solvent. The composition optionally comprises up to about 2% by weight of water. The composition preferably comprises greater than 6% of the dicarboxylic acid, more preferably greater than 6% and than about 15% of the dicarboxylic acid, even more preferably comprises about 8% to about 10% of the dicarboxylic acid and most preferably comprises about 9% of the dicarboxylic acid. The second solvent is preferably selected from propylene glycol monobutyl ether, propylene glycol monopropyl ether and diethylene glycol monomethyl ether and is most preferably propylene glycol monobutyl ether. The first organic solvent is preferably selected from the group consisting of isopropanol, n-propanol and benzyl alcohol and is most preferably isopropanol. Preferred amounts of the first organic solvent are in the range of from about 25% to about 75% by weight and preferred amounts of the second organic solvent are from about 10% to about 35% by weight. Preferably, the ratio of the first organic solvent to the second organic solvent is about 3:1. Preferred dicarboxylic acids are adipic acid, pimelic acid, suberic acid, azelaic acid and sebacic acid with pimelic acid being particularly preferred.
The invention also involves the application of the new flux composition to soldering processes used to mount electronic components, such as chips, chip carrier modules, resistors, capacitors, etc., onto PCBs.
Further objectives and advantages will become apparent from a consideration of the description and examples.